Pack the sliced red onions into a clean quart jar.

Combine the vinegar, water, sugar, salt and bay leaf in a small saucepan and bring to a boil, stirring until the sugar dissolves.

Pour the hot brine over the onions until fully covered.

Let cool on the counter for 10 minutes, then seal and refrigerate for at least 30 minutes, until the onions turn bright pink and taste tangy rather than sharp.

Lift out with a fork or slotted spoon for serving, keeping the jar chilled between refills.